Run the game using the "-opengl" launch option. For example, for vanilla SRB2, I execute "srb2win.exe -opengl" (without quotes) where SRB2 is installed.

If you run SRB2 in OpenGL through the included batch file ("SRB2 - OpenGL.bat"), you can change it to run the Top-Down mod if you copy that file, rename it if you want to, and use a raw text editor (like Notepad or Notepad++) to change "srb2win.exe" to "srb2td.exe" (or whatever the file-name for the Top-Down mod is) and then save it. (Make sure it's saved as a .bat file, not a .txt file.)